Measuring and predicting the appearance of objects under different illumination and viewing conditions is critical for many applications in computer graphics and computer vision. An important component of appearance information is the Bi-Directional Reflectance Distribution Function (BRDF), which is used to describe geometrical reflectance properties of a surface. The BRDF is defined as the ratio of reflected radiance exiting from a surface in a particular direction Ωe=(θe, φe), to the irradiance incident on the surface from direction Ωi=(θi, φi), for a particular wavelength λ, and may be written as fr(Ωi, Ωe, λ)=(dL(Ωe, λ))/(dE(Ωi, λ)).
BRDF capture, and in particular, fast acquisition of high-resolution BRDF values is challenging. The BRDF is often dependent on light wavelength and structural and optical properties of the surface being measured, such as light scattering, shadowing, light transmission, reflection, absorption, and emission by surface elements and facets. BRDF is usually integrated across a small patch, such that micro-texture is embedded within the BRDF, but not global orientation.
Most existing BRDF capture methods require a large, sophisticated setup with a camera and mirrors, and some also suffer from occlusions. Some methods require specially shaped material samples, while others require that the sample be placed inside the measurement device. Such systems cannot be used easily outside of a lab for data acquisition in the field. Furthermore, because the dynamic range of BRDFs is typically quite large, projector-camera systems must take multiple exposures in order to capture high dynamic rate measurements, limiting their maximum measurement rates.
FIG. 1 shows a gonioreflectometer 100, which is a conventional device used to capture BRDF values by moving the light source 102, the camera 104, and the object 106. The gonioreflectometer 100 is cumbersome, and since it only measures one point at a time, it takes a long time to capture the full range of the BRDF.
FIGS. 2 and 3 illustrate two examples of alternate techniques for capturing BRDF values. It is recognized that many other techniques may also be used, and those illustrated are merely representative examples.
FIG. 2 illustrates an alternate technique, known as the “Ward technique”, for capturing BRDF. The “Ward technique” 200 uses a mirror 202 and a camera 204 to capture multiple views of the sample object 206 in parallel (and in one case, to also reflect the light 208 from different directions).
FIG. 3 illustrates another alternate technique, known as the “Marschner technique”, for capturing BRDF. The “Marschner technique” 300 uses a specially formed homogeneous sample 302 to capture different orientations. The techniques illustrated in FIG. 2 and FIG. 3 could conceivably be combined if the sample is shaped as a sphere and is of a homogeneous material.
While providing good spatial resolution of the reflected light, these alternate techniques also have various limitations. The “Ward technique” 200, for example, requires mechanical motion of the light source 206, and requires that the sample 206 fit within the device. The “Marschner technique” 300 requires mechanical motion of the camera 304, and requires that the sample 302 be homogeneous and specially prepared. Both of these alternate techniques require a relatively large equipment setup, suitable for laboratory testing, but not suitable for portable or field use.